Hot runner molding

What advantages are there to using a hot runner mold rather than one that has cold runners ... 

The part taken from the mold is, in most cases, a finished product ready to be packed and shipped or ready to be used as a part of an assembled unit. In contrast to metal forming, there is very little if any wasted material in injection molding. For cold runner TP systems most runners and sprues are reground and reused. By using hot runner molds, the sprue and runner systems remain in a melted state in the mold and become part of the next finished part (Chapter 17). The hot runners can be thought of as an extension of the plasticizing chamber. 

Runners are the ehannels through which the polymer melt is fed into the mold cavities from the eyhnder nozzle. In a multicavity mold, it is necessary to fill all the mold cavities simultaneously and uniformly. Control of the size of the runners provides a means of controlling the flow resistance and balaneing the flow into the mold cavities. In most multicavity molds, the runners form part of the mold flame. Consequently, the ejected part is accompanied by the runner system, which must be removed and, in the ease of thermoplastics, reground for reuse. The use of the hot runner mold whereby the runner ehannels are heated to keep the polymer in the molten state, eliminates this need for plastic runner separation and avoids possible generation of scrap material. With proper machine operation, a hot rurmer mold requires a smaller amoimt of melt per shot than an equivalent cold runner mold, leading to redueed injeetion time and faster cycles. 

Typically, the selection of gate location and the decision between cold runner molds (two-plate or three-plate molds) or hot runner molds are important to the product design when considering the mold cost and the plastic flow within the mold. Today, about 90% of all molds built are still two-plate molds, which are considerably cheaper than three-plate or hot runner molds. For many reasons, in most cases, hot runner molds are better, but these questions must always be asked ... 

Distinguish between Injection Molding, Hot runner Molding, Compression Molding, Blow Molding, and Fiber Drawing. 

One can see from Figure 1.10 that a three-plate mold operation necessitates removal of the runner and sprue system, which must be reground, and the material reused. It is possible, however, to eliminate the runner system completely by keeping the material in a fluid state. This mold is called a hot-runner mold. The material is kept fluid by the hot-ruimer manifold, which is heated with electric cartridges. 

A hot-runner mold that is both heated and insulated an insulated mold is a hot-runner mold that does not contain heaters. 

The precise controller and gating mechanism of a hot-runner mold. 

Many molds, particularly those used for engineering thermoplastics, run at relatively high temperatures, for example, 80 C (176° F). If the mold is not insulated then, heat losses to the atmosphere, and to the machine, can easily equal those lost by the injection cylinder. So, insulate the mold from the platens and, if possible, insulate the outside surfaces of the mold. If a hot runner mold is being considered then, try and reduce heat exchanges between the hot runner parts and the cold, component-forming parts. 

Hot-runner mold (insulated-runner mold) n. An injection mold for thermoplastics in which the runners and sometimes the secondary sprues are insulated from the chilled cavity plate so that they remain hot during the entire cycle. The plastic in the runners remains molten and is not ejected with the molded part, thus avoiding the normal handling, grinding, and reprocessing of sprues and runners. 

In this technique, a rotary mold is used and parts are ejected as a continuous chain, which is later broken up or fed to machines for further processing. (See Fig. C.5.) The entire process can be completely automated for small precision parts and, with the use of hot runner molds, is said to result in considerable savings over conventional methods. 

Molds are also classified by their runner systems. In a cold-runner mold, the sprue and runner solidify and are ejected with the part. Insulated runners have much larger diameters than standard runners. As a result, the outer part of the runner solidifies while the center remains fluid. This reduces the scrap generated during molding and facilitates the changing of materials or colors. Insulated-runner molds are more difficult to operate because the gate tends to freeze off. With hot-runner molds, the runner is always in the melt state. 

Keywords cavity, demolding, design aspects, EUROMAP, gas assisted injection molding, hot runner, mold construction, mold design, part ejection, robot, runner, sprue, three-plate mold, two-plate mold. 

If multicavity molds must be built to meet product demand and the part must be center-gated (eg, round gears) there are two tooling options a three-plate mold or a hot runner mold. A three-plate mold provides a rimner path as shown in Figure 15, which allows for center-gating multiple parts. 

Figure 11-30. Gate breaking and clipping are low-cost removal methods when the part design permits locating a recessed gate in a hidden area, as in this handle. This type of problem is eliminated by using hot runner molds for TPs and cold runner molds for TSs.

In a hot runner mold, the runner is contained in a manifold which has electrical heating cartridges. The heated runner keeps the GRTP in a fluid condition (Fig. 7-4). 

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